In a variable capacity piston pump installed in a vehicle such as a construction machine, a discharge flow of the piston pump is regulated by moving a servo piston of a servo regulator in order to vary a tilt angle of a swash plate of the piston pump.
JP2009-243425A discloses a servo regulator that controls an operation of a servo piston by driving a spool valve using a solenoid to regulate an oil pressure in pressure chambers facing respective ends of the servo piston.